1. Field
The present invention relates generally to data communication, and more specifically to techniques for determining a non-uniform distribution of data rates to be used for multiple data streams to be transmitted via multiple transmission channels of a multi-channel communication system, e.g., a multiple-input multiple-output (MIMO) system.
2. Background
In a wireless communication system, an RF modulated signal from a transmitter may reach a receiver via a number of propagation paths. The characteristics of the propagation paths typically vary over time due to a number of factors such as fading and multipath. To provide diversity against deleterious path effects and improve performance, multiple transmit and receive antennas may be used. If the propagation paths between the transmit and receive antennas are linearly independent (i.e., a transmission on one path is not formed as a linear combination of the transmissions on the other paths), which is generally true to at least an extent, then the likelihood of correctly receiving a data transmission increases as the number of antennas increases. Generally, diversity increases and performance improves as the number of transmit and receive antennas increases.
A multiple-input multiple-output (MIMO) communication system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, with NSxe2x89xa6min{NT,NR}. Each of the NS independent channels may also be referred to as a spatial subchannel (or a transmission channel) of the MIMO channel and corresponds to a dimension. The MIMO system can provide improved performance (e.g., increased transmission capacity) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
For a full-rank MIMO channel, where NS=NTxe2x89xa6NR, an independent data stream may be transmitted from each of the NT transmit antennas. The transmitted data streams may experience different channel conditions (e.g., different fading and multipath effects) and may achieve different signal-to-noise-and-interference ratios (SNRs) for a given amount of transmit power. Moreover, if successive interference cancellation processing is used at the receiver to recover the transmitted data streams (described below), then different SNRs may be achieved for the data streams depending on the specific order in which the data streams are recovered. Consequently, different data rates may be supported by different data streams, depending on their achieved SNRs. Since the channel conditions typically vary with time, the data rate supported by each data stream also varies with time.
If the characteristics of the MIMO channel (e.g., the achieved SNRs for the data streams) are known at the transmitter, then the transmitter may be able to determine a particular data rate and coding and modulation scheme for each data stream such that an acceptable level of performance (e.g., one percent packet error rate) may be achieved for the data stream. However, for some MIMO systems, this information is not available at the transmitter. Instead, what may be available is very limited amount of information regarding, for example, the operating SNR for the MIMO channel, which may be defined as the expected SNR for all data streams at the receiver. In this case, the transmitter would need to determine the proper data rate and coding and modulation scheme for each data stream based on this limited information.
There is therefore a need in the art for techniques to determine a set of data rates for multiple data streams to achieve high performance when limited information is available at the transmitter for the MIMO channel.
Techniques are provided herein to provide improved performance for a MIMO system when channel state information indicative of the current channel conditions is not available at the transmitter. In an aspect, a non-uniform distribution of data rates is used for the transmitted data streams. The data rates may be selected to achieve (1) a specified overall spectral efficiency with a lower minimum xe2x80x9creceivedxe2x80x9d SNR (described below) or (2) a higher overall spectral efficiency for a specified received SNR. A specific scheme for achieving each of the above objectives is provided herein.
In a specific embodiment that may be used to achieve the first objective noted above, a method is provided for determining data rates to be used for a number of data streams to be transmitted via a number of transmission channels in a multi-channel communication system (e.g., one data stream may be transmitted over each transmit antenna in a MIMO system). In accordance with the method, the required SNR for each of a number of data rates to be used for the data streams is initially determined. At least two of the data rates are unequal. The xe2x80x9ceffectivexe2x80x9d SNR (described below) for each data stream is also determined based on the received SNR and successive interference cancellation processing at the receiver (also described below) to recover the data streams. The required SNR for each data stream is then compared against the effective SNR for the data stream. The data rates are deemed to be supported if the required SNR for each data stream is less than or equal to the effective SNR for the data stream. A number of sets of data rates may be evaluated, and the rate set associated with the minimum received SNR may be selected for use for the data streams.
In a specific embodiment that may be used to achieve the second objective noted above, a method is provided for determining data rates for a number of data streams to be transmitted via a number of transmission channels (e.g., transmit antennas) in a multi-channel (e.g., MIMO) communication system. In accordance with the method, the received SNR is initially determined. This received SNR may be specified for the system or may be estimated based on measurements at the receiver and periodically provided to the transmitter. The effective SNR for each data stream is also determined based on the received SNR and successive interference cancellation processing at the receiver. The data rate for each data stream is then determined based on the effective SNR for the data stream, with at least two of the data rates being unequal.
Various aspects and embodiments of the invention are described in further detail below. The invention further provides methods, processors, transmitter units, receiver units, base stations, terminals, systems, and other apparatuses and elements that implement various aspects, embodiments, and features of the invention, as described in further detail below.